Voice coil motor actuators are commonly used in magnetic disk file apparatus. The voice coil is attached to a carriage assembly and is located in an air gap defined in a magnet assembly. A number of magnetic read/write heads are mounted on the carriage. A signal, in the form of a current, is supplied to the coil and interacts with the magnetic field set up by the magnet assembly, causing motion of the carriage and therefore of the heads across the surface of disk or disks within the disk file.
The motive force on the carriage results in an equal and opposite reaction force on the magnet assembly, which can excite unwanted resonances in the disk file e.g. in the spindle of the disk motor. This reaction can therefore lead to relative motion between the disk and the read/write heads and to instability in a closed loop head positioning servo system.
The principle of absorbing the reaction force in voice coil motor actuators is well known; IBM Technical Disclosure Bulletin Vol. 19 no. 9 pp 3561-3562 describes an actuator in which the effects of the reaction force are reduced by floating the voice coil motor magnet structure. The magnet structure is supported at both ends by ring flexure assemblies in which a layer of damping material is laminated between two metal sheets. The flexures are mounted onto rigid connector bars which are in turn mounted on the disk file baseplate. In this way, the magnet assembly is effectively isolated from the magnetic disks. U.S. Pat. No. 3 699 555 describes a similar system.
In the design of voice coil motor actuators, it is also common practice to employ a shorted turn, made from a non-magnetic conductive material, to improve the current rise time in the voice coil. European patent application 0 222 139 describes a shorted turn in the form of a copper sleeve which is in contact with the magnet assembly and located in an air gap defined by the magnet assembly. The shorted turn abuts against two rubber rings used as end stops to prevent overtravel of the carriage. U.S. Pat. No. 4 661 729 also describes a cylindrical shorted turn which, in this case, is located on the cylindrical surfaces of two shoulders at the front and rear of the magnet assembly, the shorted turn abutting against one of these shoulders. A seal is formed between the shorted turn and the shoulders by `O` rings located in two grooves in the shoulders.